A device for the distrubution of pulverulent products suspended in a gas on a substrate, such as a ribbon of glass is known by U.S. Pat. No. 4,230,271. The pulverulent products suspended in gas are distributed through a distribution slot placed above the glass. The distribution slot is the lower end of a nozzle which comprises a cavity extending over the entire length of the nozzle and exhibits a venturi shaped cross section. The cavity is fed with a pulverulent product suspended in a gas by a plurality of elementary ducts of the same length resulting from the subdivision of a single pulverulent product feeding duct. Also, the nozzle comprises a broad approximately parallelepiped homogenization chamber that exhibits a venturi-shaped cavity and extends over the entire length of the nozzle. The homogenization chamber also receives gas under pressure to create the turbulences required to homogenize the mixture of the pulverulent product and gas. The homogenization chamber comprises a narrow passage extending over the entire length of the nozzle that exhibits an expanding section and then a converging section which terminates at the distribution slot.
Although the nozzle provides desirable results, it is sensitive to clogging and periodically requires cleaning to continue operating correctly. This periodic cleaning results in a loss of production. In addition, while such nozzles have been made with distribution slot lengths of 250 to 650 mm, to coat glass ribbons several meters wide there must be several identical nozzles positioned end to end. In such circumstances, it is extremely difficult to balance the various nozzles to assure a regular and uniform distribution of pulverulent product over the entire width of the glass. It would be preferable to use a single nozzle of greater length, for example a length at least equal to that of the ribbon of float glass which generally reaches more than 3 m, but it has been found that the uniformity of the resultant layer is reduced. This results in unacceptable variations in color of the deposited layer.
To eliminate these drawbacks, U.S. Pat. No. 4,562,095 proposed a process and a device for distribution of a pulverulent product on a substrate, whose length corresponds to the width of the substrate, while also providing a uniform distribution of the pulverulent product in time and space. The object of that patent was directed to the forming of a stream of a pulverulent product suspended in a gas in the shape of a blade, directly above the substrate and over a length at least equal to the width of the substrate to be coated. In addition, to maintain the stream of the pulverulent product in a continuous flow in the direction of the substrate over its entire length, gas currents are introduced into this stream to create turbulence in an attempt to homogenize the mixture of gas and the pulverulent product as the mixture is directed toward the substrate. Finally, to uniformly accelerate the movement of the pulverulent product gas mixture toward the substrate, this mixture is entrained in additional gas currents introduced from the sidewalls of the device toward the direction of the substrate. According to that patent, the homogenization and acceleration can take place in one and the same step.
U.S. Pat. No. 4,562,095 specifically describes a nozzle as a device for the distribution of the pulverulent product whose length can correspond to the width of the substrate to be coated. This nozzle comprises a longitudinal cavity whose walls converge uniformly in the direction of the substrate to be coated, so that the zones of homogenization and acceleration converge. The upper part of the nozzle comprises an element for covering the cavity which is pierced with an orifice which makes possible the feeding, through separate ducts, of the pulverulent product suspended in a gas which is called a primary gas. The ducts for feeding the pulverulent product primary gas mixture are not constructed in an airtight manner in the orifice, and this allows the introduction of air from the ambient atmosphere to be introduced into the inside cavity of the nozzle.
In the upper part of the nozzle, the longitudinal walls of the cavity and the covering element delimit two continuous slots which extend over the entire length of the nozzle through which additional gases under pressure are introduced. These gases are called secondary gases. The secondary gas is injected at a speed much greater than that of the primary gas in which the pulverulent product is suspended. The increased speed makes it possible to accelerate the displacement of the pulverulent product toward the distribution slot of the nozzle and facilitates the uniform distribution of the injected gas flow over the entire length of the nozzle. The secondary gas also entrains gas or air from the ambient atmosphere (induced gas or air), thus making possible the formation of turbulence which facilitates the homogenization of the pulverulent product gas mixture.
This device exhibits certain advantages, in particular, the injected secondary gas and the induced air enter at the same level as the end of the pulverulent product feeding ducts. This makes it possible to eliminate the dangers of depositing pulverulent product on the walls of the cavity of the nozzle or of driving the pulverulent product back through the orifice of the upper part of the nozzle.
The aim is, however, to improve still further the homogenization of the powder-gas mixture and to make the distribution of this mixture still more uniform throughout the whole length of the nozzle.